Extra-framework aluminum species

The previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

Dealuminated samples were obtained by hydrothermal treatment of calcined MCM-22 (Si/Al = 15) at different temperatures (673, 773, 873 K) for 2-24 h under a saturated flow of a nitrogen/steam mixture (flow rate of 200 ml min"1). These steamed samples were further treated with 6N HNO3 solution at 353 K for 4 h in order to remove the extra-framework aluminum species. 

Al MAS NMR has been demonstrated to be an invaluable tool for the zeoHte sdentist It provides a simple and direct way to quantify the proportions of A1 in four [Al(4)j, five [Al(5)j and six [Al(6)j coordinations. Quantitative determination of these species is an important issue in catalysis, and major effort is devoted on this topic. As mentioned already, for A1 only the central transition (-i-half to —half selective exdtation ) is detected. The central transition is unaffected by first order quadmpolar interaction, but the presence of second order effects causes broadening and complicates the quantitation of the A1 species. Usually hydrated samples and short radiofrequency pulses are employed for quantitative determination of framework and extra framework aluminum species. It is uncertain whether hydration changes the coordination of A1 species. Certain extra framework A1 can have very large quadmpolar interactions resulting in very broad lines ( NMR invisible ) [155, 202]. Unlike Si NMR, Al has a short relaxation time due to its quadmpolar nature, and the Al NMR spectrum with good signal to noise can be obtained in a relatively short time. 

When the thermokinetic parameter was plotted versus the amount of NH3 adsorbed for samples of H-ZSM5 (Si/Al = 10.3) pretreated at 400 and 800°C it was found that the maximum time constant is higher for the sample pretreated at 800°C than for that pretreated at 400°C [103]. In fact, the increase of the pretreatment temperature caused dealumination extra-framework aluminum species were created that restricted the access to the channels and created diffusional limitations. 

Macedo et al. [227] studied HY zeolites dealuminated by steaming, and found that the strength of intermediate sites decreased with increasing dealumination for Si/Al ratios varying from 8 to greater than 100. For comparison, isomorphously substituted HY, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [227], This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

The microcalorimetry of NH3 adsorption coupled with infrared spectroscopy was used to study the effect of the synthesis medium (OH or F ) on the nature and amount of acid sites present in Al,Si-MFl zeolites [103]. Both techniques revealed that H-MFl (F ) with Si/Al < 30 contained extra-framework aluminum species. Such species were responsible for the presence of Lewis acid sites and poisoning of the Brpnsted acidity. In contrast, MFl (F ) characterized by Si/Al > 30 presented the same behavior as H-MFl (OH ). 

The simultaneous investigation of the methanol conversion on weakly dealuminated zeolite HZSM-5 by C CF MAS NMR and UV/Vis spectroscopy has shown that the first cyclic compounds and carbenium ions are formed even at 413 K. This result is in agreement with UV/Vis investigations of the methanol conversion on dealuminated zeolite HZSM-5 performed by Karge et al (303). It is probably that extra-framework aluminum species acting as Lewis acid sites are responsible for the formation of hydrocarbons and carbenium ions at low reaction temperatures. NMR spectroscopy allows the identification of alkyl signals in more detail, and UV/Vis spectroscopy gives hints to the formation of low amounts of cyclic compounds and carbenium ions. 

Cu-MCM-41 and Cu-Al-MCM-41 samples have been obtained by ion exchange of the MCM-41 and Al-MCM-41 matrices prepared by hexadecyltrimethylammonium cloride, tetraethyl orthosilicate, aluminum isopropoxide and an ammonia solution. The aluminum concentration affects the MCM-41 textural properties and large amount of extra-framework aluminum species are supposed to be present in Al-MCM-41 with Si/Al = 30. Cu-MCM-41 and Cu-Al-MCM-41 catalysts have been tested for NO selective catalytic reduction by propane in the presence of oxygen, in comparison with microporous Cu-S-1 and Cu-ZSM-5 catalysts with similar copper loading and Si/Al atomic ratio. Cu-Al-MCM-41 catalysts are less active and selective with respect to the Cu-ZSM-5 catalysts indicating that they are not suitable for NO abatement reactions. 

The synthesis of Al-MCM-41 has been reported by several authors and the distribution between framework and extra-framework aluminum species has been found to depend strongly on the silica and aluminum sources, the nature of templating surfactant and synthesis conditions [Ref 2 and references therein]. In this contribution we investigate a simple preparation method based on hexadecyltrimethylammonium cloride, tetraethyl orthosilicate, aluminum isopropoxide and an ammonia solution. 

In hydrothermally treated NH4 Y samples, both sorts of bridging OH groups are accessible to pyridine, but the interaction with the OH groups of the line at 5.2 ppm, corresponding to the LF infrared band, remains weaker than with those of the 4.2 ppm resonance line, corresponding to the HF band (22.). It is well established that in such samples extra-framework aluminum species are present giving rise to new OH bands at 3600 and 3670 cm"1... 

The different performances of catalysts (A) and (B) was elucidated by characterization of these materials [29]. Al and Si MAS NMR showed that after treatment with 0.01 M HCl most of the amorphous silica material is removed from the parent catalyst (A), leaving extra-framework aluminum species also created by the steaming procedure [29]. This can be readily understood, because the solubility of silica is maximum at pH 2 [34]. It is believed that the silica species blocked most of the catalytically active centers, i. e. the highly dispersed Lewis acidic, extra-framework alumina sites, which seem to be partly bonded to the zeo-litic framework of the starting material (A). The EFA species are not, therefore, leached out. 

Zeolites are important refining catalysts for two reasons. The first, and most important, is the presence of strong acid sites. Two types of acid sites are present in zeolites. The first are Brpnsted acid sites, shown in Figure 10.6. These are protons that act as charge-compensating cations for framework aluminum. The second are Lewis acid sites, which are less well defined than Brpnsted sites and involve extra-framework aluminum species formed by removing framework aluminum (often due to steam). Both of these are extremely active catalytic centers, which can activate relatively inert substrates such as normal alkanes. 

For most zeoUtes, when NH3 is used as the probe molecule at a given temperature, the time needed to estabhsh thermal equihbrium after each dose at first increases with increasing adsorbed amount, passes through a maximum, then decreases rapidly and finally reaches a value close to the time constant of the calorimeter. For example. Fig. 3 shows the time constant (in seconds) versus the amount of NH3 adsorbed for samples of H-ZSM-5 (Si/Al = 10.3) pretreated at 673 K or 1073 K and possessing a very small particle size (0.02 to 0.05 xm). The maximum time constant is higher for the sample pretreated at 1073 K than for the sample pretreated at 673 K, because increasing the pretreatment temperature causes dealumination, i.e. creates extra-framework aluminum species which restrict the access to the channels and creates diffusional limitations. The time constant of the calorimeter was close to 300 s. The heat transfer is determined by the mass transfer which becomes slower. 

For the dealuminated zeolites produced by exposing Na-Y to an SiCU stream, an enhanced concentration of extra-framework aluminum species (EFAL) inside the zeolite crystal was discovered, and heats > 130kjmol were observed in the SiCU-treated samples. This was not the case on faujasites dealuminated with EDTA, which showed a lower concentration of strong... 

Extra-Framework Aluminum Species Displayed With TRAPDOR... 

Jiao J, Kanellopoulos J, Wang W, Ray SS, Foerster H, Freude D, et al. Characterization of framework and extra-framework aluminum species in non-hydrated zeolites Y by Al spin-echo, high-speed MAS, and MQMAS NMR spectroscopy at = 9.4 to 17.6 T. Phys Chem Chem Phys 2005 7 3221-6. 

Sulikowski [29] calculated the fractal surface dimension of NH4,Na-Y zeoHte steamed at 550°C from adsorption data of short-chain alcohols. This property of matter being a quantitative measure of surface roughness was found to be 2.25, which points to a significant deviation from a smooth surface due to the formation of extra-framework aluminum species and mesopores at the external surface. 

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